1. Field of the Invention
The present invention relates to a method for manufacturing a spark plug, and to apparatus and devices adapted for practicing the method.
2. Description of the Related Art
A related art spark plug for use in an internal combustion engine includes a metal shell having a tool engaging portion and a threaded mounting portion, and an insulator inserted into a through hole which penetrates the metal shell in an axial direction. In such a related art spark plug, an airtight seal is provided between the insulator and the metal shell so as to prevent gas generated in the internal combustion engine from leaking through a gap between the insulator and the metal shell. In order to seal the space between the insulator and the metal shell, a technique is employed in which the upper end portion of the metal shell is crimped onto an outer circumference of the insulator and a portion of the metal shell is compressively deformed (see, e.g., Patent Document 1)    Patent Document 1: JP-A-2007-141868
3. Problems to be Solved by the Invention
Meanwhile, there has been a demand for a reduction in the size and diameter of spark plugs in order to enhance the degree of freedom in the design of internal combustion engines. However, since the mechanical strength of the insulator is decreased due to a reduction in size and diameter, it is difficult to provide an airtight seal between the insulator and the metal shell. In the above technique, for example, if a portion of the metal shell is too compressively deformed, the insulator may become damaged. If the compressive deformation on a portion of the metal shell is insufficient, the sealing properties between the insulator and the metal shell may be insufficient.